Hybrid antibodies are antibodies or aggregates of antibodies which are specific for two different antigens. Hybrid antibodies can comprise a single antibody or fragment having a bispecific antigen binding region (two different variable regions) or aggregates of two or more antibodies of different specificities.
Different methods of preparing hybrid antibodies have been reported. Auditore-Hargreaves teaches processes for preparing hybrid antibodies by generating "half molecules" from two parent antibodies and subsequently associating different half molecules. See U.S. Pat. Nos. 4,470,925 (1984) and 4,479,895 (1984). Using this process, various hybrid antibodies were prepared with specificities or horseradish peroxidase, glucose oxidase and theophylline.
Reading describes production of antibodies having binding specificities for two desired antigens using a quadroma cell or a trioma cell. See U.S. Pat. No. 4,474,893 (1984). The quadroma cell is the fusion product of two different hybridoma cells, each of which produce an antibody with a different specificity. A trioma cell is the fusion product of a hybridoma and a lymphocyte which produces antibodies with two different binding specificities.
Segal et al. describe target specific cross-linked heteroantibodies which are used as cytotoxic agents in U.S. Patent 4,676,980 (1987) . Staerz et al. (1986), PNAS, 83:1453-1457, teach the use of a hybrid antibody that can focus effective T cell activity and Milstein et al. (1983), Nature, 305: 537-539 , describe the use of hybrid antibodies in immunohistochemistry.
Raso et al., Cancer Research, 41:2073-2078 (1981) disclose the use of hybrid antibodies with dual specificity for the plant toxin, ricin, and immunoglobulin-bearing target cells. The hybrid antibodies were constructed in vitro and the attachment of the hybrid antibody-ricin complex to the human target cells was observed using fluorescein labeled antibodies. Upon binding, the human target cells were selectively killed by the hybrid-delivered toxin.
Prior to the use of hybrid antibodies, chemical crosslinking or nonspecific absorption methods were used to couple drugs and/or toxins to antibody carriers . These agents possess certain limitations due to the nature of the linkage. The linkage may alter the drug or toxin such that the therapeutic or toxic activity is reduced. Moreover, cleavage of the covalent bond may be rate-limiting for the action of toxin inside the cell.
The use of hybrid antibodies obviated some of the problems encountered with chemical crosslinking or non-specific absorption methods; however new problems were created. Because the drug or toxin is bound to an antibody, the therapeutic or toxic activity is generally inhibited. Hybrid antibody-delivered toxins or drugs are inactive when bound to the antibody and only become active upon release. However, the hybrid antibodies currently available have no mechanism for releasing the toxin or drug from the respective antibody binding region when the hybrid antibody reaches the target site or the interior of the cell. Instead, they rely on fortuitous dissociation. As a result, relatively large quantities of hybrid antibodies containing drugs or toxins must be administered, because only a small amount of the drug or toxin will dissociate and become active.